UNITED STATES ENVIRONMENTAL PROTECTION AGENCY RESEARCH TRIANGLE PARK, NC 27711



## HAY 102013 OFFICE OF AIR QUALITY PLANNING AND STANDARDS 

### MEMORANDUM
SUBJECT:



### FROM:

TO: Interim Guidance to Implement Requirements for the Treatment ofAir Quality Monitoring Data Influenced by

**E~ce pti**  **na 1** 7vents Stephen D. Page, Director

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This memorandum and its attachments clarifykeyprovisions ofthe 2007 Exceptional Events Rule (EER) to respond to questions and issues that have arisen since the rule was promulgated. The interim guidance in this memorandum and the attachments, along with examples ofapproved demonstrations on the EPA's website1 (collectively the "interim exceptional events guidance materials"), are provided to help ensure an efficient and effective process to make determinations regarding air quality data affected by exceptional events. Ourintent is to streamlineprocesses and reduce costs for air agencies2preparing requests and the EPA offices reviewing these submittals. The EPA is neither setting new policies nor raising novel issues through this guidance.
We first released draft exceptional events implementation guidance documents to air agencies, Federal Land Managers, and to otherparties as requested, in May of2011. We incorporated some ofthe commenters' feedback into the revised draft guidance documents, which were made available for broad public review in a July 6, 2012, FederalRegisterNotice ofAvailability (77 FR 39959) and in the associated docket (Docket ID No. EPA-HQ-OAR-2011-0887). An accounting ofthe comment and response process from the 2011 preliminaryreview is documented in the docketed response to comments document.3

One important differencebetween the interim exceptional events guidance documents released today and the draft guidance documents made available to the publicviathe July 2012 FederalRegister Notice ofAvailabilityis the EPA's clarification that this interim guidance is intended to provide recommendations and to indicate the EPA's current thinking on exceptional event issues, rather than conveying requirements not already stated in the Clean AirAct (CAA) and the EER. Additionally, the EPA revised the interim guidance materials to correct typographical errors, to make editorial changes to reflect the December 14, 2012, promulgation ofthe fine particle (PM2

.s) National Ambient Air Quality Standards (NAAQS), and to reflect terminology consistent with the ongoing ozone NAAQS review.

1 Additional information and examples ofexceptional event submissions and bestpractice components canbe found at the EPA's Exceptional Events website located at http://www.epa.gov/ttnlanalysis/exevents.htm. 2 References to "airagencies" are meant to include state, local, and tribal air agencies responsible for implementing the EER. 3 Response to Comment document titled, "Responses to SignificantFirst-Round Comments onthe Draft Guidance Documents onthe Implementation ofthe Exceptional Events Rule."
Internet Address (URL) • http://www.epa.gov Recycled/Recyclable • Printed

**w~h** Vegetable Oil Based Inks on Recycled Paper (Minimum 25% Poslconsumer)

### 2

With this memorandum and its attachments, the EPA's Office ofAir and Radiation is simultaneously announcing its intent to pursuerevisions to the 2007 EER. We anticipate proposing these rule revisions in late 2013 or early 2014 and finalizing a revised rule in late 2014 or early 2015. As we move forward with a notice and commentrulemaking process, there will be an opportunity for all interested parties, including those that commented during the 2012 public commentperiod, to raise any issues or concerns. The EPA's regional offices should use the interim guidance as we undertake rule revisions because it is consistent with the EER and the guidance already provided inthe preamble to the rule.
The interim guidance materials are based onthe following principles: 1. Air agencies should notbeheld accountable for exceedances due to exceptional events that were beyond their control at the time ofthe event. 2. Itis desirable to implement reasonable controls to protectpublic health. 4 3. Clear expectations will enable the EPA and other air agencies to bettermanage resources related to the exceptional events process.
These interim guidance materials identify the four independent criteria on which exclusion ofeventaffected data depends, describe the administrative process and associated timing for submittal and review ofdemonstrations, provide answers to frequently asked questions, and provide previously reviewed demonstrations and bestpractice components. The EPA recognizes the challenges that air agencies face inpreparing exceptional event demonstration packages. The EPA also recognizes the limited resources ofthe air agencies that prepare and submit exceptional event demonstrationpackages and ofthe EPA regional offices that review these demonstration packages. One ofthe EPA's goals in developing exceptional event implementation guidance is to establish clear expectations to enable affected air agencies to better manageresources as theyprepare the documentation required under the EER. These interim guidance documents and the exceptional events website present examples to illustrate specific points. The example analyses and level ofdetail are not necessarilyneeded for all demonstrations. Submitters should prepare and submit the appropriate level ofsupporting documentation, which will vary on a case-by-case basis using the weight-of-evidence approach. The EPA anticipates that the resources needed to prepare (and review) packages will decrease as we continue to identifyways to streamline the process and continue to build our database ofexample demonstrations and analyses. In addition, extreme exceptional events mayjustify a more limited demonstration package. Exceptional Event Rule Provisions
On March 22, 2007, the EPA promulgated the "Treatment ofData Influenced byExceptional Events; Final Rule" (72 FR 13560) pursuant to the 2005 amendment ofCAA Section 319. This rule, known as the EER, superseded the EPA's previous natural events guidance and those sections ofthe interim fire policy document that address exceptional events.5 The EER created a regulatoryprocess codified at 40

4 With respect to exceptional events, Section 319 ofthe CAA states the following guiding principles (among others); (i) the principle that protection ofpublic health is the highest priority 

### *** (iv) the principle that each State must take necessary measures to safeguard public healthregardless ofthe source ofthe air pollution 5Previous guidance and policy documents that either implied or documented the need for identifying data affected by an

exceptional event include: i) "Guideline for Interpretation ofAir Quality Standards," U.S. EPA, OAQPS No. 1.2-008, Revised February 1977. ii) "Guideline On the Identification and Use ofAir Quality Data Affected byExceptional Events" (the Exceptional Events Policy), U.S. EPA, OAQPS, July 1986.3
CFRparts 50 and 51 (50.1, 50.14 and 51.930). These regulatory sections contain definitions, procedural requirements, requirements for air agency demonstrations, and criteria for EPA approval for the exclusion ofair quality data from regulatory decisions under the EER.
The definition ofan exceptional event at 40 CFR §50.1(j) repeats the CAA definition, whichprovides that an exceptional event is one that affects air quality, is notreasonably controllable orpreventable, and is caused byhuman activitythat is unlikelyto recur at a particular location or a natural event. 40 CFR §50.1(k) further defines a natural event as one inwhichhuman activityplays little orno direct causal role. Additional requirements in 40 CFR §50.14(a)(2) and (b)(1) identifythat an air agencymust demonstrate "a clear causal relationship betweenthemeasured exceedance or violation ofsuch standard and the event" and that "an exceptional event caused a specific air pollution concentration in excess of
one ormore national ambient air quality standards." The rule further requires at 40 CFR §50.14(c)(3)(iv) that the demonstration tojustifydata exclusion shall provide evidence that the event is associated with a measured concentration in excess ofnormal historical fluctuations, includingbackground, and evidence that there would havebeen no exceedance or violationbut for the event.
Treatment ofTechnical Criteria for Exclusion ofData Affected byEvents
When considered together, the EERprovisions summarized above identify the following six elements that air agencies must address whenrequesting that the EPA exclude event-related concentrations from regulatory determinations:
• the event affected air quality
• the event was notreasonably controllable orpreventable
• the event was causedbyhuman activitythat is unlikelyto recur at a particular location, or was a natural event • there exists a clear causal relationship between the specific event and the monitored concentration
• the event is associated with a measured concentrationin excess ofnormal historical fluctuations includingbackground
• there would havebeen no exceedance orviolationbut for the event
Inreviewing exceptional events demonstrationpackages, the EPA has found that the following EER elements, along withhistorical fluctuations, play a significantrole in the air agencies' supporting documentation:
1. not reasonably controllable orpreventable 2. ifthe event was causedbyhuman activity, thathuman activity is unlikelyto recur at a particular location6

iii) "Areas AffectedbyPMlO Natural Events" (the PMlO Natural Events Policy), memorandum from MaryD. Nichols, AssistantAdministrator for Air and Radiation, to the EPAregional offices, May 30, 1996. iv) "The Interim Air QualityPolicy onWildland and PrescribedFires" (the Interim Fire Policy), memorandum from Richard D. Wilson, Acting Assistant Administrator for Air and Radiation, to the EPAregional administrators, May 15, 1998. v) "Guideline on DataHandling Conventions for the PM NAAQS," U.S. EPA, OAQPS, EPA-454/R-98-0 17, December 1998. 6 Neither the statutorynorregulatory defmition of"exceptional event" requires a demonstration of"unlikelyto recur" for natural events.4
3. clear causal relationship between specific event and monitored concentration 4. no exceedance or violation but for the event7

As described inthe interim guidance materials, the EPA's technical review ofa demonstration package therefore focuses on these elements. While the EERrequires and the EPA anticipates complete demonstration packages to contain narrative and evidence supporting all six elements, the EPA's positionis that these four elements represent distinct facts for air agencies to demonstrate for the EPA to concur on an event claim.8 Ifan event is natural, then the second element (e.g., human activityunlikely to recur) is generally not considered in a demonstration review. In the case ofan event that is initiated by
a natural process, such as a volcano orhigh wind dust event, the event would be considered a natural event ifsources are entirely natural or contributing anthropogenic sources are reasonably controlled. 9 This concept is explained inmore detail inAttachment 2, the interim High Winds Guidance document.
The EPA recognizes the inherent linkages between all six elements and expects that some sections ofa demonstration package (e.g., affects air quality, natural event) mayrepeat orrefer to other sections of the demonstrationpackage (e.g., clear causal relationship, but for). Further, each potential event can have varied and differing characteristics, and thus would usually necessitate a case-specific demonstration and evaluation. Therefore, the EPA would use a "weight-of-evidence" approach in evaluating each element within an exceptional event demonstrationpackage.
In the interim guidance materials, the requirement that the event was notreasonably controllable or preventable, which is part ofthe definition ofan exceptional event inboth the CAA and the EER, would
meanthat ifa set ofcontrol measures should reasonably have been inplace for contributing sources at the time ofthe event, then it musthave been inplace for the event to qualify as an exceptional event under the EER. Whether a set ofcontrols should reasonablyhavebeen inplace is event-, time-, and place-dependent, and involvesjudgementbythe air agency whenpreparing the demonstration and by the EPA when reviewing the demonstration. The EER requirement for reasonable control applies to all events but is more complicated for high wind dust events because these events typicallyincludeboth natural and anthropogenic sources ofdust. In contrast, an event such as a lightning-induced wildfire

7 Criteria 1, 3, and 4 onthis list, along with historical fluctuations, are considered "independent elements" inthe interim High Winds Guidance document. 8 The EPA generally does not consider the two remaining elements, "affects air quality" and "historical fluctuations," to represent "distinctfacts." The EPAbelieves that the "affects air quality'' element is generally satisfied once the air agency satisfies the clear causal and historical fluctuations showings. While the "historical fluctuations" element is considered an independent element, it also plays an important role in the "clear causal relationship" and "no exceedance but for" demonstrations. The EPA will review air agency submissions using a weight-of-evidence approach. The air agency's role in satisfying this element is to provide appropriate analyses and statistics comparing the event-affected concentration to normal historical fluctuations and conclude that the provided data show that the event was in excess ofnormal historical fluctuations. The EPA will review the information providedbythe air agency. ''Normal historical fluctuations" will generallybe defined bythose days without events for the previous years. The EPA acknowledges that natural events canrecur and still be eligible for exclusion under the EER; therefore, events do not necessarilyhave to be rare to satisfy this element. However, inmost cases, the EPA anticipates that less conclusive "historical fluctuations" comparisons will likely indicate less conclusive "clear causal relationship" and/or "no exceedance but for" relationships. 9 The EPAwill generally considerhuman activity to have played little orno direct role in causing emissions ofthe dust generated byhigh wind for purposes ofthe regulatory defmition of"natural event" ifcontributing anthropogenic sources of dust are reasonably controlled, regardless ofthe amount ofdust coming from these reasonably controlled anthropogenic sources, and thus the event couldbe considered a natural event. In such cases, the EPAbelieves that it would generally be a reasonable interpretation ofits regulations to fmd that the anthropogenic source had "little" direct causal role. If anthropogenic sources ofwindblown dust that are reasonably controllable but that did nothave those reasonable controls applied at the time ofthe high wind event have contributed significantly to a measured concentration, the event would notbe considered a natural event. See footnote 11, 72 FR 13566.

### 5 generally does not include an anthropogenic contribution to the event. 10 Among other factors to consider, reasonableness would need to bejudged in light ofthe technical information available to the air agency at the time the event occurred. The EPA anticipates that nonattainment areas alreadyhave the technical information needed to reasonably control anthropogenic sources in theirjurisdiction. Generally, the EPA does not expect areas classified as attainment, unclassifiable, ormaintenance for a NAAQS to have the same level ofcontrols as areas that are nonattainrnent for the same NAAQS. Also, ifan area has beenrecently designated to nonattainment buthas not yet been required to implement controls, the EPA will expect the level ofcontrols that is appropriate for the planning stage. Regardless ofattainment status ornatural/anthropogenic source contribution, each demonstration package should address the question ofreasonable controls. In general, reasonable controls would not include any control on emissions-generating activity outside ofthe state ortribal boundaries ofthe state (or tribal lands) within which the concentration at issue was monitored. As withthe other elements, whether an event was notreasonably controllable or preventable would be evaluated on a case-by-case basis. Ifand when the EPA takes a regulatory action that hinges on a decision to exclude dataunder the EER, the EPA will consider and appropriately respond to anypublic comments on whether the event was "not reasonably controllable orpreventable." Timing ofEERDemonstration Package Submittal and Review
The EPA understands that the initial identification ofdata affected by exceptional events and the subsequent preparation, submittal, and review ofdemonstration packages is a resource intensive process. Delays inprocessing and making decisions on submitted packages create regulatoryuncertainty and potentiallyincrease the workload for boththe submitting air agency and the EPA. In addition, the backlog ofpending actions makes retrieval ofdata to support new submittals potentiallymore difficult. Further, air agencies and the EPA often face timelines by which theymust make regulatory decisions that can be affected bythe inclusion or exclusion ofevent-affected data.
The EPA will work with air agencies as theyprepare complete demonstrationpackages that meet the requirements ofthe EER. In an effort to streamline this

**identification~** preparation, submittal, and review process, the EPA has developed the following interim guidelines.
1. Identification ofdata affected by exceptional events 

### in the EPA's Air Quality System (AQS)

- The EPA is aware that air agencies routinely review their air quality monitoring data, which may result in the identification ofcertain databeing affected by an exceptional event. Although air agencies mayflag any data in AQS that they wish to flag, the EPA encourages air agencies to flag only data that might have a regulatory consequence and for which an approvable demonstration is likely. 11 Should air agencies wish to flag values for informational purposes, the EPA prefers that theyuse the AQS flags intended for this purpose.

10 The EPArecognizes that wildfires and emissions from wildfires are generally notreasonable to prevent or control. Although the EER requires documentation ofthis criterion for all event types, the EPA believes that it will generallybe sufficient for air agencies to provide a briefstatement to document the "notreasonably controllable orpreventable" criterion for wildfires. See Question 20b ofthe Interim ExceptionalEvents RuleFrequentlyAsked Questions document for example language. 11 Air agencies should place flags and an initial event description inAQS either in accordance with the special schedules promulgated with new orrevised NAAQS or in accordance with the general AQS data submission schedules (i.e., within 90 days ofthe end ofthe previous quarter) but not later than July 1st ofthe calendar year following the event in which the flagged measurement occurred. Note that for data certificationpurposes, we recommended flagging data prior to submitting data certification (May 1 51). ·6
2a. Air agency submittal ofletter ofintentto submit'a package 

### (optional)- To promote early communication, the EPA suggests that air agencies provide a letter ofintent to submit a demonstration package for flagged data in AQS as soon as possible, ifpossible within 12 months from the event occurrence, after the air agency identifies the event(s) as being significant. 12 This initial notification can assistboth the air agency and the EPA inthe planning and prioritization process.
2b. Air agency notification ofintent to submit apackage 

### (optional)

- Air agencies choosing not to submit a letter ofintent are still encouraged to contact their EPA regional office more informally to alert it ofthe forthcoming demonstration submittal.
3. EPA response to air agency letter ofintent- The EPA anticipates responding to the air agency's letter ofintent within 60 days ofreceipt. The EPA response will provide the regional office's best assessment ofthe prioritythat canbe given to the submission once received and any casespecific advice the EPA mayhave to · offer for the preparation ofthe demonstration.
4. Air agency submittal ofexceptional event demonstration packages 13

- Air agencies should prepare a technical demonstration package, taking into account the information in the EPA's guidance documents, which shows that a particular air qualitymonitored value(s) was influenced by an exceptional event. The EPA acknowledges that extreme exceptional events mayjustify
more limited demonstrationpackages. Air agencies thatbelieve their demonstration packages are tied to near-term regulatory actions should submit their demonstration packages well in advance ofthe regulatory deadline. Air agencies should also identify the relationship between the exceptional event-related flagged data and the anticipated regulatory action inthe cover letter that accompanies their initial submittal package to the reviewing EPA regional office.
5. EPA prioritization ofsubmitted demonstration packages- The EPA will generally give priority to exceptional event determinations that may affect near-term regulatory decisions, such as state implementation plan (SIP) submittal actions, NAAQS designations, and clean data findings.

6. EPAreview ofprioritized demonstration packages- The EPA generally intends to conduct its initial review ofa submitted exceptional event demonstrationpackage within 120 days ofreceipt. Following this initial review, the EPA will generally send a letter to the submitting air agency that includes a completeness determination and/or a request for additional information, a date by which the supplemental information should be submitted (ifapplicable), and an indicator ofthe timing ofthe EPA's final review. The EPA encourages air agencies to provide supplemental information for which the EPA asks. The EPA anticipates a 60-dayresponse time for states to

12 The Letter ofIntent is an optional step and the EPArecognizes that air agencies may need additional time to prepare and submit demonstrationpackages particularly where the basis ofthe exclusion is violating an annual standard or a 3-year design value. Similarly, an air agency could consider submitting an annual letter ofintent ifannual submittal makes sense for
resource planning or for historically seasonal events. Ifan air agency decides to submit a letter ofintent, the EPA recommends that it be submitted as expeditiously as possible after the air agency identifies the event or events as having significance. 13 The general schedule in the EER allows air agencies to submitpackages up to 3 years following the end ofthe calendar quarter in which the event occurred, or 12 months prior to the date that a regulatory decision mustbe made bythe EPA. When the EPApromulgates a new or revised NAAQS, we may change this schedule to allow air agencies to flag and submit documentation for data relevant to the new/revisedNAAQS.7 provide additional requested information. 14 The EPA intends to make a decision regarding event concurrence as expeditiously as·necessary ifrequired bya near-term regulatory action, butno later than 18 months following submittal ofa complete package. Determinations on exceptional event demonstrations do not constitute final agency action until they are relied upon in a regulatory decision such as a finding ofattainment or nonattainment which will be conducted through notice-and-comment rulemaking procedures.
Interim Exceptional Events Rule Frequently Asked Questions Document (Attachment 1)
The "Interim Exceptional Events Rule Frequently Asked Questions" document (the interim Q&A document) provides interim responses to questions that have arisen since the EPA promulgated the EER. The questions are grouped into six broad areas. The EPA encourages those involved in flagging data and preparing demonstration packages to review the questions and answers and to provide input regarding their usefulness and appropriateness and regarding additional questions which need answers. The following bullets identifykeypoints ofinterest in the interim Q&A document:

• Natural events, such as volcanic eruptions, do not have to beinfrequent to qualify as exceptional events under the EER (see Question 1 ofthe interim Q&A document). Frequent events with natural triggers that have a contribution from anthropogenic activities that are reasonably controlled couldbe eligible "exceptional" events, provided the events meet the demonstration requirements for the technical criteria.

• The EER does not prohibit air agencies from flagging individual concentration values below the level ofthe NAAQS. However, in general, only such data that contribute to a violation ofthe NAAQS are excludable. Questions 29-31 describe the few, limited situations in which concentration values below the level ofthe NAAQS contribute to**viol~tions** ofthe NAAQS.

• An event that an air agencyhas concluded is associated with a measured concentration "in excess ofnormal historical fluctuations" will be reviewed using a weight-of-evidence approach. The comparison ofthe measured concentration to normal historical concentrations will also influence how much information is needed to successfullymeet other technical elements. For example, when the observed concentration is high compared to historical concentrations, the EPA mayneed less additional evidence to demonstrate the "but for" finding. Questions 1-5 provide recommendations for showing how the observed concentration compares to the distribution ofhistorical concentrations.

• Question 6 describes types ofevidence that could be submitted as part ofa demonstration showing that an ozone exceedance would not have occurred but for the effect ofa fire event. In particular, statistical orphotochemical dispersion model predictions ofthe ozone concentration that would have occurred inthe absence ofthe fire wouldbe a relevant type ofevidence, provided the demonstration package is transparent about the technical basis for the model and its uncertainties. Also, as noted below, the EPA intends to .develop a separate draft document to provide guidance inpreparing demonstrations for wildfire-related events that influence ambient ozone concentrations.

14The EPA recognizes that air agencies may need more than 60 days to prepare and submit some types ofsupplemental information. The EPA will work with air agencies on supplemental timeframes; however, the mandatorytiming ofthe EPA actions may limit the response time the EPA allows.

### 8

• Not everynatural or infrequent anthropogenic event that affects air quality is a true "exceptional event" under the definition ofthat term in the EER. Ambient data affected by an event that does notmeet the "but for" criterion cannotbe excluded under the authority ofthe EER even ifin all otherrespects the event meets the definition ofan exceptional event. The EER does not address datahandling associated with events that are not considered "exceptional" under the EER, and does not provide the EPA with authority to exclude such data. Yet, the event-related concentration could still influence design values. An air agency incorporating the event-related concentration in a design value used for a prospective attainment demonstration might seem to need more emission reductions to attain theNAAQS byits attainment deadline than is actually the case. The EPA plans to more formally address this topic on a pollutant/NAAQS basis, the first ofwhich will be ozone guidance inthe preamble ofa soon-to-be-proposed rulemaking on SIP requirements for areas designated nonattainment for the 2008 ozone NAAQS. Until the planned guidance for a pollutant and NAAQS ofinterest is issued, air agencies should consult with their EPA regional office ifthey face this situation. The EPA further discusses this issue in Question 13.

• To remove anypossible confusion, the passages ofthe preamble that were declared to be a legal nullitybythe court thatreviewed the EER are specificallyidentified in Question 20. 15 While air agencies canriot rely onthese passages as the EPA guidance on interpretation ofthe EER, this interim guidance overview document and its attachments are consistent with those sections.

• The EPA identifies in Question 28a currently existing mechanisms that air agencies can use at various points in the exceptional events process to resolve disagreements regarding nonconcurrence on submittal packages.
Interim High Winds Guidance Document (Attachment 2)
The attached "Interim Guidance on the Preparation ofDemonstrations in Support ofRequests to Exclude Ambient Air Quality DataAffected byHighWinds Under the Exceptional Events Rule" (the High Winds Guidance document) is a resource for air agencies when flagging data and preparing demonstrations packages for high wind dust events that have affected particulate matter concentrations

- both particles less than or equal to 10 micrometers (J.Lm) in diameter (PM 10) and PM2
.5. The interim document applies the provisions ofthe EER, the general guidance conveyed in this memorandum, and the general guidance in the interim Q&A document to the particular situation ofa high wind dust event. While the document is specific to high wind dust events, it outlines how the EPA generally intends to implement the preparation and review process for exceptional events and, therefore, mayhave relevance for other types ofexceptional events. The following are some ofthe highlights ofthe interim High Winds Guidance document:

• In nonattainment areas, a reference point for considering what constitutes reasonable control of wind-blown dust during high wind events would be the set ofmeasures that are identified as Reasonably Available Control Measures (RACM) or Best Available Control Measures (BACM) in the approved SIPs ofother areas with similar wind-blown dust conditions, depending on area classification. U.S. Department ofAgriculture (USDA) 

### I Natural Resources Conservation Service (NRCS)-approved conservation management practices designed to effectivelyreduce fugitive dust air emissions and prevent loss ofsoil during high winds could also be considered.

15 SeeNRDCv. EPA, No. 07-1151 (D.C. Cir. 3/20/09).

### 9
All dust-related control measures and/or dust suppression measures in an area's own approved SIP should be considered part ofthe set ofcontrols that wouldhave been reasonable to have been inplace at the time ofthe event. The assessment ofwhether an event was not reasonably controllable will bemade on a case-by-casebasis. Like other elements included in an exceptional events demonstration, when the EPA takes a regulatory action that hinges on a decision to exclude dataunder the EER, the EPAwill consider and appropriately respond to anypublic comments onwhether the event was "notreasonably controllable orpreventable."

• Reasonable controls generally would not need to beimplemented for wind-blown dust from undisturbed natural landscapes orpreviously disturbed landscapes that are being allowed to return to natural conditions.

• Forpurposes ofqualifying for the exclusion ofdata affected bywind events with sustained wind speeds above 25 miles perhour (or above another threshold determined to be appropriate for a particular area), the demonstration ofreasonable controls appliedto disturbed landscapes and other anthropogenic sources ofdust could be less rigorous because: (1) the contribution from natural undisturbed lands is likely to behigh and, (2) at suchhigh wind speeds many available controls would have been ineffective in significantlyreducing wind-generated dust emissions.

• In response to commenter feedback, the EPA has added the optionalprospective controls analysis, which air agencies canprepare to document existing controls and facilitate the EPA's review and evaluation ofthe notreasonably controllable and preventable criterion. In the prospective controls analysis, the air agency would provide information on attainment status, identifynatural and anthropogenic windblown dust sources and emissions, provide the status of SIP submittals and their implementation (ifapplicable), and identifythe wind speed up to which the collective windblown dust controls are expected to be effective. Air agencies would submit their prospective controls analysis in advance ofan air agency submittal and the EPA review of any specific demonstration submittal, with a letter ofintent, orwith their demonstrationpackage submittal. The EPA review and approval ofcontrols and an appropriate high wind threshold would typicallybe effective for a minimum ofthree years.

• Ifthe EPA has approved a SIP containing wind-blown dust controls within the pastthree years, then the submitting air agencyhas the option ofusing their current, implemented SIP-approved controls and specifying a high wind threshold to which the controls are expected to be effective to constitute the set ofcontrols that would have beenreasonable to implement.

• Air agencies are encouraged to workwiththe EPA regional offices to develop High Wind Action Plans, whichneed not be incorporated into the SIP, as a wayto develop a mutual understanding ofwhat additional controls are reasonable to implement in light offoreseeable high wind conditions. Similarto a Natural Events Action Plan16

, the optional High WindAction Plari is a mechanism to implement necessary controls on newly-identified sources needing reasonable controls such that the EPA could consider future high wind events not reasonably controllable or preventable.

160nMay30, 1996, Mary D. Nichols, Assistant Administrator for Air and Radiation issued a memorandum to the EPA regional offices titled, "Areas Affected byPM 10 Natural Events." The policy, known as the PM 10 Natural Events Policy, or sirnply the Natural Events Policy, set forth procedures for protecting public health through the development ofa Natural Events Action Plan, which implements Best Available Control Measures for human-generatedparticulate emissions in areas that could violate the PM 10 NAAQS due to natural events. Promulgation ofthe EER superseded the Natural Events Policy

.10

On-line Availability ofExceptional Event Packages and Best Practice Components
To assist air agencies in deciding what type and howmuch evidence/technical analysis to include in their demonstration packages, the EPA has developed apublic website at http://www.epa.gov/ttnlanalysis/exevents.htm that contains EPA-approved demonstration packages and links to bestpractice components. The EPA developed this website to provide examples to illustrate specific points; the example analyses and level ofrigor are not necessarily needed for all demonstrations. The website will continue.to evolve as air agencies submit, and the EPA reviews, additional demonstration packages.
Draft Guidance Under Development
The EPAis currently developing a separate draft guidance document addressing the preparation of demonstrations to support data exclusionrequests for wildfire-related events that mayhave affected
ozone concentrations. We anticipate preparingthis guidance within the same timeframe as the EER revisions with draft guidance available inlate 2013/early 2014 and final guidance available in late 2014/early 2015. We will provide an opportunity for stakeholder input on this guidance.
Conclusion Regional offices should use the interim guidance provided in this overview document and its attachments as we undertake rule revisions, because itis consistent with the EER and the guidance alreadyprovided inthe preamble to therule.
Staffin the EPA's Office ofAir Quality Planning and Standards are available for assistance and consultation. Forinterim guidance-related questions, please contact BethW. Palma at (919) 541-5432 or palma.elizabeth@epa.gov.
Attachments: 1. Interim Exceptional Events Rule FrequentlyAsked Questions 2. Interim Guidance on the Preparation ofDemonstrations in Support ofRequests to Exclude Ambient Air QualityDataAffected byHigh Winds Under the Exceptional Events Rule